The invention relates to a device and method for optical, contactless vibration measurement of a vibrating object.
Such a method utilizes and such a device comprises a laser Doppler vibrometer with a laser as light source for a laser beam. This laser beam is split into a measurement beam and a reference beam in a first beam splitter arrangement. Either the measurement beam or the reference beam experiences a frequency shift, for which an acousto-optic modulator, in particular a Bragg cell, is usually used. The measurement beam is directed to an object to be measured and scattered back or reflected from there. A second beam splitter arrangement combines the measurement beam scattered back from the vibrating object with the reference beam and superposes the two beams, resulting in an interference signal. The superposed measurement and reference beams are fed to a detector, which generates an electric measurement signal from the interference signal.
Laser Doppler vibrometers are able to perform contactless measurements up to the megahertz range of vibrations of objects, in particular of object surfaces. This opens up applications when measuring vibrations of very small and light structures, such as in micro-systems technology. However, it is also possible to measure vibrations in the air and in fluids with the aid of a laser Doppler vibrometer. Here, the frequency of the measurement beam is modulated by the movement of the object surface to be measured as a result of the Doppler Effect during reflection. Since the laser emits coherent light, an interference signal emerges from the superposition of the measurement beam, which is frequency modulated by the object movement, and the reference beam, which remains unchanged, from which interference signal it is possible to derive the speed of the object. Hence the vibration speed of the surface of the object to be measured is acquired.
Since one of the two parts of the laser beam is frequency shifted in the presently utilized laser Doppler vibrometer and hence a heterodyne vibrometer is present, a modulation frequency of the interference signal is generated which renders it possible to determine not only the current speed of the surface of the object to be measured, but also the sign, i.e. the movement direction, and so the vibration movement of the object to be measured can be acquired uniquely using the heterodyne laser Doppler vibrometer.
The shift in frequency of the reference or measurement beam, which, as mentioned above, is typically achieved by an acousto-optic modulator (Bragg cell), is only a fraction of the frequency of the laser light. Use is typically made of a helium-neon laser (He—Ne), the frequency of which lies at 4.74×1014 Hz. The shift in frequency of the reference or measurement beam is typically merely 40 MHz. However, thermal influences on the laser, and there in particular on the optical resonator thereof, lead to changes in the resonator length which could change the frequency of the laser beam by a multiple of the frequency shift caused by the Bragg cell. In the case of a helium-neon laser, an increase in temperature by only 0.1° C. typically leads to a shift in frequency of the laser beam by 300 MHz.
This causes particular problems if use is made of several laser Doppler vibrometers, which simultaneously direct the measurement beams thereof to a region of a vibrating object to be measured, for example in order to be able to establish three-dimensional vibrations. This is because if the frequencies of the measurement beams are too close, a crosstalk effect can occur here, i.e. the backscattered measurement beam from one vibrometer which at the same time reaches another vibrometer can falsify the measurement there or render it impossible to take a measurement. Since the center frequency of each He—Ne laser is determined by a well-defined atomic electron level, the center frequencies of a number of lasers do not differ. If the frequencies of the measurement beams of several such vibrometers are shifted due to thermal effects, such crosstalk effects occur randomly again and again.
In order to meet the aforementioned problems, it is known to stabilize the frequency of the laser employed in the laser Doppler vibrometer. Here, this laser is provided with a control loop, which, in particular, acts on the length of the optical resonator in order to compensate for temperature-induced changes in length and/or frequency shifts. At the same time, it is ensured that the laser only emits one active mode which is used for the measurement. A known frequency stabilization consists in using an unpolarized laser without a preferred polarization direction and with two active modes, taking one of the two modes by means of a polarization beam splitter and using this mode as a controlled variable.
However, such a frequency stabilized laser is disadvantageous in that it reacts very sensitively to unwanted reflections and other stray light reaching the optical resonator. Moreover, the maximum laser power of such a frequency stabilized laser is not available for carrying out a measurement. This is particularly very disadvantageous if a measurement device of the type in question should make full use of the emitted light power permitted by the selected laser class in order to achieve maximum measurement accuracy.
The aforementioned disadvantage of reduced power of a frequency stabilized laser can be avoided if a laser is employed in a device and a method of the type in question, in which the active modes are all used for the measurement. This is preferably brought about by virtue of a polarization filter being employed within the optical resonator of the laser, which polarization filter brings all active modes of the laser to the same polarization such that, firstly, the full laser power is available for the measurement and, secondly, the laser becomes very much less sensitive against back reflection and stray light as a result of the polarization filter. However, according to current knowledge, frequency stabilization of this laser is then ruled out.
Without frequency stabilization, the problems which were described at the outset, in particular in the context of using several laser Doppler vibrometers, in turn emerge. But even in the case of a vibration measurement using only one laser Doppler vibrometer, the laser of which emits more than one active mode, it is possible for the signal strength of the interference signal to collapse at certain values of a temperature induced frequency shift such that a measurement is no longer possible. This is the case, in particular, if two active modes are emitted which have approximately equal amplitudes and which interfere destructively. Vibration measurements can fail in this manner, namely if, for example, temperature influences from the surroundings lead to the laser reaching a mode state in which a measurement is not possible.
Proceeding from this prior art, the present invention is based on the object of providing a device and a method of the type mentioned at the outset, by means of which a laser of a laser Doppler vibrometer is stabilized in respect of its frequency, in particular for avoiding crosstalk effects in the case of measurements with two or more laser Doppler vibrometers, and the laser Doppler vibrometer can nevertheless be operated with almost maximum signal strength.